Messages in this thread |  | | Date | Mon, 9 Apr 2018 11:47:07 +0100 | From | Will Deacon <> | Subject | Re: [PATCH 10/10] locking/qspinlock: Elide back-to-back RELEASE operations with smp_wmb() |
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Hi Boqun,
On Sat, Apr 07, 2018 at 01:47:11PM +0800, Boqun Feng wrote: > On Thu, Apr 05, 2018 at 05:59:07PM +0100, Will Deacon wrote: > > @@ -340,12 +341,17 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) > > goto release; > > > > /* > > + * Ensure that the initialisation of @node is complete before we > > + * publish the updated tail and potentially link @node into the > > I think it might be better if we mention exactly where we "publish the > updated tail" and "link @node", how about: > > * publish the update tail via xchg_tail() and potentially link > * @node into the waitqueue via WRITE_ONCE(->next,..) below. > > and also add comments below like: > > > + * waitqueue. > > + */ > > + smp_wmb(); > > + > > + /* > > * We have already touched the queueing cacheline; don't bother with > > * pending stuff. > > * > > * p,*,* -> n,*,* > > - * > > - * RELEASE, such that the stores to @node must be complete. > > * publish the updated tail > > > */ > > old = xchg_tail(lock, tail); > > next = NULL; > > @@ -356,15 +362,7 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) > > */ > > if (old & _Q_TAIL_MASK) { > > prev = decode_tail(old); > > - > > - /* > > - * We must ensure that the stores to @node are observed before > > - * the write to prev->next. The address dependency from > > - * xchg_tail is not sufficient to ensure this because the read > > - * component of xchg_tail is unordered with respect to the > > - * initialisation of @node. > > - */ > > - smp_store_release(&prev->next, node); > > /* Eventually link @node to the wait queue */ > > Thoughts?
I'll make some changes along these lines for v2. Thanks!
Will
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